1. Field of the Invention
The present invention relates to an objective lens for regenerating a video disc, a digital audio disc or the like, and particularly to an objective lens which can decrease a deterioration in performance resulting from an error of assembly.
2. Description of the Prior Art
In objective lenses used for regeneration of a video disc, a digital audio disc or the like, a high contrast up to about 1,000 pieces/mm of space frequency is required, and if a semi-conductor laser is used as a light source, it is necessary to control an aberration on an optical axis within a diffraction limit where N.A. is 0.45 to 0.5.
However, in the case the objective lens is directly driven to effect tracking, the optical axis of the objective lens is always kept vertically to the track and is moved parallel to the disc surface, and therefore, correction of out-of-axis aberration is rarely necessary in terms of principle.
In order to directly drive the objective lens, the objective lens must be as small and light as possible, and to this end, an objective lens whose focal distance is 4 to 5 mm is desired. If the working distance is not sufficient, there unfavorably gives rise to an inconvenience in that the objective lens comes to contact with the disc surface to damage the disc and the objective lens.
To be used for such a purpose, an objective lens is known which comprises a first cemented lens having a positive lens and a negative lens arranged in named order from an object side and a second positive meniscus lens with a convex surface directed towards the object (For example, Japanese Patent Application Laid-Open No. 55-4068). However, if the first lens and the second lens becomes eccentric, the aberration is greatly deteriorated, as a consequence of which it is necessary to make a clearance between the lens and a lens mount small or to make adjustment after assembled. In case of the former, a machine tool of high precision becomes necessary and in case of the latter, the number of steps of adjustment becomes necessary.